1. Field of the Invention
The present invention relates to display apparatuses and portable terminals, and more particularly, to a display apparatus which uses a reference-voltage-selection-type D/A conversion circuit in a digital-type horizontal driving circuit that writes a display signal into each pixel of a display section, and a portable terminal to which the display apparatus is mounted as a screen display section.
2. Description of the Related Art
In the field of flat-panel-type display apparatuses, typical of which are liquid-crystal display apparatuses and electroluminescence (EL) display apparatuses, so-called driving-circuit-united-type display apparatuses have been developed in order to make the frames of the panels smaller and make the panels thinner. In the driving-circuit-united-type display apparatuses, a display section in which pixels are arranged in a matrix manner and peripheral driving circuits for driving the display section are mounted on a transparent, insulating substrate as a unit.
The peripheral driving circuits of the display apparatuses include a vertical driving circuit for selecting pixels in the display section in units of lines and a horizontal driving circuit for writing display data into each pixel in the selected line, as typical driving circuits. There are an analog-type horizontal driving circuit and a digital-type horizontal driving circuit. The digital-type horizontal driving circuit includes a D/A conversion circuit for converting a digital display signal to an analog display signal. As D/A conversion circuits, reference-voltage-selection-type D/A conversion circuits are known, in which a plurality of reference voltages corresponding to the number of gradation levels is generated by a reference-voltage generation circuit, and a reference voltage corresponding to a digital display signal is selected among the plurality of reference voltages and output as an analog display signal.
FIG. 9 shows a basic structure of the reference-voltage generation circuit. The reference-voltage generation circuit 100 according to the basic structure uses a resistor division (voltages divided by resistors). More specifically, when the number of gradation levels is “n”, the voltage between a first reference potential VA and a second reference potential VB is divided by (n−1) resistors, R1 to Rn−1, connected in series. With this, (n−2) reference voltages, V1 to Vn−2, are obtained at voltage-division points. When a reference voltage V0 is set to the reference potential VA, and a reference voltage Vn−1 is set to the reference potential VB, a total of n reference voltages, V0 to Vn−1, are generated.
The reference-voltage generation circuit 100, shown in FIG. 9, has a structure used when it is mounted on liquid-crystal display apparatuses. In liquid-crystal display apparatuses, alternating-current (AC) inversion driving is employed which inverts the polarity of a display signal at a certain interval, in order to prevent the resistivity (resistance unique to a material) of the liquid crystal and others from deteriorating, the deterioration being caused by the continuous application of a direct-current (DC) voltage having the same polarity to the liquid crystal. To this end, switches SW1 to SW4 are turned on (closed) and off (opened) by timing pulses φ1 and φ2 generated alternately in synchronization with AC inversion, in the reference-voltage generation circuit 100.
In the reference-voltage generation circuit 100, when the timing pulse φ1 is generated at certain inversion timing of AC inversion, since the switches SW1 and SW4 are turned on, a positive power-supply voltage VCC is given as the first reference potential VA, and a negative power-supply voltage VSS (for example, a ground level) is given as the second reference potential VB. When the timing pulse φ2 is generated at the next inversion timing, since the switches SW2 and SW3 are turned on, the negative power-supply voltage VSS is given as the first reference potential VA, and the positive power-supply voltage VCC is given as the second reference potential VB.
When a driving-circuit-united-type display apparatus is structured, since various driving circuits are mounted on a substrate having a limited size, a restriction is given to the position of the reference-voltage generation circuit 100 on the substrate. Especially when horizontal driving circuits are arranged above and below a display section, the reference-voltage generation circuit 100 needs to be disposed at a position which has an equal distance from the above and below horizontal driving circuits, that is, inevitably, an intermediate position adjacent to the display section, on the substrate.
An input pad section for inputting from the outside of the substrate into the inside of the substrate, display data, a master clock MCK, a horizontal synchronization signal Hsync, a vertical synchronization signal Vsync, and the power-supply voltages VCC and VSS is provided at an end of the substrate on either the above side or the below side of the display section. For this reason, especially when the reference-voltage generation circuit 100 is arranged at the intermediate position adjacent to the display section, the power-supply lines of the power-supply voltages VCC and VSS need to path through long on the substrate from the input pad section to the reference-voltage generation circuit 100, and their wiring lengths are long. This arrangement of the power-supply lines on the substrate makes the wiring resistance of the power-supply lines large.
When the wiring resistor of the VCC power-supply line is called Rvcc and the wiring resistor of the VSS power-supply line is called Rvss, as shown in FIG. 10, the reference potentials VA and VB are reduced by a voltage α equal to Iref×Rvcc or a voltage β equal to Iref×Rvss due to the existence of the wiring resistors Rvcc and Rvss, where Iref indicates DC current flowing through the resistors R1 to Rn−1, as shown in a waveform view of FIG. 11. The wiring resistors Rvcc and Rvss also include the switching resistors of the switches SW1 to SW4.
The reference voltage V0, which is equal to the reference potential VA, is used for a black level (black voltage), and the reference voltage Vn−1, which is equal to the reference potential VB, is used for a white level (white voltage). Therefore, when the reference potentials VA and VB are reduced due to the arrangement of the VCC and VSS power-supply lines in the-substrate, since the black level or the white level is reduced, the contrast ratio decreases and the image quality is strikingly reduced. In normally-white-mode liquid-crystal display apparatuses, the reduction of the black level especially reduces the image quality.